This disclosure relates generally to X-ray radiation sources, and more particularly to a liquid anode radiation source.
The various imaging technologies constitute an accepted and integral part of our everyday life. Applying various types of high-intensity radiation sources (e.g. neutron sources, X-ray sources, etc.) these imaging technologies are widely used in non-destructive quality control (see the neutron diffraction material structure testing methods), security engineering (see airport radioscopic screening) or medical diagnostics.
The imaging technologies based on the use of X-rays constitute a significant group of medical imaging technologies, including but not limited to for example computer tomography (CT) or μ-CT, as well as various methods of radiography and mammography. For these diagnostic methods, the part from 1 to 300 keV photon energy of the electro-magnetic radiation is used which is usually produced by means of an X-ray tube. The X-ray beam is practically produced in such a way that the electron beam of appropriate energy is set on a specific region (the focal spot) on the internal metal surface of the X-ray tube, called the anode. The electrons impacting the material of the specific region of the anode are slowed down within a very short time as a result of which one part of their kinetic energy forms X-ray radiation, while the other part (more than 99%) is used for the warming of the anode in the form of heat.
The warming of the anode significantly influences the amount of the tube current to be applied in the X-ray tube, as well as for a given tube current the smallest size of the mentioned focal spot in the case of use when the solid anode will not yet melt. If the anode is overheated, then it will result in the melting of the anode material in the focal spot and the anode surface in the focal spot will become uneven. Because of this, the intensity of the X-ray radiation coming from the focal spot will decrease. In order to eliminate or reduce the problem caused by warming of the anode, the anode of solid material anode X-ray tubes are made of metals having very high melting points, usually wolfram (W) or molybdenum (Mo) on a design that turns around an axis in order for the heat load on the anode to distribute on a greater surface.
For imaging, proper detection of the information carrier (for example, the X-ray beam coming out of the system to be diagnosed) is necessary. The detectors serving this purpose are known to professionals. For taking an image of proper quality, that is, for detecting with the required noise level, it is necessary to ensure exposure of a given extent on the detector. The combination of the required exposure and the exposure time characteristic of the irradiation of the system to be diagnosed will determine the minimum tube current to be used for the X-ray tube. The exposure of the detector is directly proportional to the product of the tube current and the exposure time. In order to reduce the extent of artifacts resulting from displacement to the minimum possible level, it is a general aim that the required exposure is reached within the shortest possible exposure time. For example, in CT applications the exposures necessary for taking each projection can be achieved on the detector in order to ensure the given image quality through the application of great tube current with small exposure times. So for a solid material anode X-ray tube operating with a focal spot of given (effective) size (usually of 0.3-1.0 mm diameter), the tube current connected to the X-ray tube has a definite maximum for avoiding the melting of the anode. If greater tube current is applied, then the anode material will melt at the focal spot. Accordingly, the melting of the anode material at the focal spot will define the shortest realizable exposure time, which is unfavorable for imaging.
Therefore, if we want to increase the maximum tube current of an X-ray tube, then we should increase the (effective) size of the focal spot. It is understood that in this case, the distance of the focal spot from the detector will also be increased in order for the contrast and spatial resolution of the image taken with the detector can be maintained. It will result in the increase of the external dimensions of the actual diagnostic imaging equipment. In other words, when the imaging equipment is operated with a given resolution, the increase of the focal spot in a given proportion and the increase of the distance of the X-ray tube focal spot from the object to be diagnosed in the same proportion will not result in the modification of the exposure affecting the detector.
To summarize, the real parameter characterizing the “goodness” of the imaging equipment containing a solid material anode X-ray tube (that is their image quality, efficiency, safety, etc. with a given radiation load) is the maximum value of the X-ray tube current falling on the unit area of the focal spot or in other words the maximum current density measured on the focal spot.
Attempts were made for replacing the solid material anode of X-ray tubes. For example, U.S. Pat. No. 4,953,191 describes an X-ray source which bombs liquid (that is melt) gallium flowing on a vertical plane metal plate with a source beam and in this way produces X-ray radiation. Prior to impacting the liquid gallium with proper speed, the source beam is led through a high-voltage accelerating space. The metal plate of the X-ray source serves for maintaining and stabilizing the flowing gallium. The movement of the liquid gallium takes place on the vertical plane metal plate, so the stabilization of the gallium stream is done on a plane surface. Consequently, the X-ray source operates only in vertical, standing position in order to prevent the gallium from “sliding down” the metal plate. The liquid gallium is kept in continuous motion that is circulated in the X-ray source by means of an electromagnetic pump. The problem of gallium entering the accelerating space is not solved, so the operability of this liquid anode radiation source is doubtful.
U.S. Pat. No. 5,052,034 discloses an X-ray source having an anode constituting the source of X-ray radiation in the form of a liquid metal on a plane-surface anode holder. For the solutions considered, the anode holder is expediently covered with gallium (Ga), indium (In), tin (Sn), or alloys of these metals. The flowing off of the liquid metal from the anode holder is prevented by the surface forces (surface tension) acting between the particles of the liquid metal and the particles of the anode holder found on the surface of the anode holder. The supply of the liquid anode material on the anode holder is provided through the condensation of the evaporating anode material. Since the surface forces are of a restricted amount, this solution practically requires the use of a horizontal anode holder. Even to a small-extent, canting of the X-ray source (and thus the anode holder) will result in the outflow of the liquid anode material from the anode holder and thereby the termination of the production of X-ray radiation. It is a further disadvantage that the flowing back of the liquid anode material into the accelerating space realized in the form of high-voltage vacuum space may easily occur which may result in the failure of the X-ray source. In another proposed solution, the (low steam-pressure) metal constituting the liquid anode is kept in continuous flow by means of a Faraday pump in a self-contained channel formed in insulation material. The bombing of the liquid anode with electrons takes place in a section of the mentioned channel in which the liquid anode material flows on a plane surface, with itself also being spread on a plane. The source beam is produced by means of a cathode placed in an airtight space separated from the anode material.
In order to avoid the problem of the anode material getting into the accelerating space of the electrons constituting the electron beam, the mechanical separation of the accelerating space and the liquid anode material by means of a sufficiently thin separation window may give a solution.
U.S. Pat. No. 6,185,277 treats such an arrangement where the high-voltage vacuum space is separated from the liquid anode by a thin electron window made from suitable material. There is a restriction placed in the liquid flow below the window. Under the influence of the restriction, the flow of the liquid anode material below the window will become turbulent, improving the cooling of the window. Cooling of the window is considered in U.S. Pat. No. 6,477,234. According to the '234 patent, the flow of another liquid is led before the window serving the introduction of the source beam, which will achieve the increased cooling of the window concerned by carrying away one part of the heat produced in the window under the influence of the source beam passing through it. Further liquid anode X-ray sources achieved with electron window are disclosed in U.S. Pat. Nos. 6,925,151 and 6,961,408. The considered solutions do not eliminate, only reduce the problem of electron window warming. As a result, such a relatively thin electron window is subject to fatigue fracture owing to the accumulated thermal and mechanical stress, as it is mentioned by U.S. Pat. No. 7,412,032 and thus, it may lead to the unforeseen failure of the X-ray source. In addition, the integration of such windows in the X-ray sources will increase the complexity of the manufacturing processes and production costs of liquid anode X-ray sources.
Therefore, there exists a need for an improved liquid anode radiation source that can operate in an optional direction in any orientation that the liquid anode radiation source has the ability of turning “upside down”.